Field of the Invention
This invention is an appratus for collecting and uniformly distributing a downflowing liquid in a liquid-vapor contact tower while allowing easy (low pressure-drop) passage of the vapors in the tower through the apparatus. The invention is related to a field of art commonly referred to as liquid redistributors. Such redistributors find wide usage in process towers in the chemical and petrochemical industries wherein it is desired to obtain intimate mixing of vapors and liquids within them.
Ordinarily the vapors in such towers rise while the liquid is falling, but in some instances both the vapor and liquid flow downwardly. This invention is applicable to both situations.
Also, such redistributors are normally used in vapor-liquid contact towers which contain beds of column packing. Usually a redistributor is placed beneath a bed of such column packing in a tower in order to collect the liquid flowing downwardly through this bed and then redistribute the liquid in a uniform manner across a second bed of column packing located beneath the redistributor. This is done because it is well known that liquid becomes maldistributed as it descends through such beds due to such phenomena as channeling in the bed and adherence of the liquid to the sides of the tower walls. Such maldistribution results in the loss of the desired intimate, uniform mixing of vapor and liquid as the liquid descends downwardly in the bed. To offset this maldistribution, redistributors are placed at strategic levels within the vapor-liquid mixing zone in the tower.
Probably the most difficult problem encountered in designing, fabricating, installing, and operating such liquid redistributors is that of managing liquid flowing downwardly in the tower at very low liquid flow rates. Normally, liquid flow rates are usually from about 2.0 to about 10.0 gallons per minute per square foot of tower horizontal cross-sectional area. It is at flow rates less than about 2.0 gallons per minute per square foot that problems arise in distributing the liquid. The important aspect of any column packing in any system is to keep all the packing at least moist so as to avoid "dry or hot spots" in the packing which the vapor contacts. It would seem that at low liquid flow rates this purpose would be best achieved by increasing the number of liquid distribution points per square foot of tower horizontal cross-section, or it would appear to do so at first blush by those not skilled in the art. However, more distribution points in turn would require even a smaller liquid flow rate through each distribution point. And smaller flow rates would mean smaller openings in the distribution points. To those skilled in the art a problem is present when one goes to smaller diameter distribution points. This problem is a plugging problem of the distribution points caused by solid particles or flecks of debris present in the liquid. These flecks tend to plug up small-diameter distribution points.
Of course, such plugging results in the redistributor doing exactly the opposite of what it is supposed to do; i.e., keeping the liquid evenly distributed across the top of the bed of column packing located immediately beneath the redistributor.
The ladder-type liquid distributor (not redistributor) disclosed in U.S. Pat. No. 4,444,696 solves this problem for distributing liquid which is collected outside the tower and then fed into the tower through the distributor. It provides distribution points whose inlet cannot be plugged by debris, and it employs a larger pressure head on the liquid to be distributed. This pressure head greatly alleviates distortion in the uniformity of the liquid distribution. This distortion in liquid distribution is ordinarily present due to undesired, but present, non-uniformity in the level of all of the distribution point inlets.
But, of course, the distributor disclosed in this patent is not a redistributor, i.e., it is not designed to collect liquid falling from above and U.S. No. 4,444,696, is incorporated by reference as if set forth at length.
A redistributor which overcomes the plugging problem is disclosed in U.S. Pat. 4,432,913, which is also incorporated by reference. However, this redistributor does not provide a way for employing a large head pressure on the distribution point inlets to improve the uniformity of liquid distribution.
It would be advantageous to have a liquid redistributor which not only overcomes the debris plugging problem, but also provided a means for producing a large pressure head on the liquid being redistributed.